One of the problems with airplanes and high-speed
flight is the heat that builds up on the airplane’s surface
because of air friction. When the SR-71 Blackbird
airplane is cruising at Mach 3.5, skin temperatures
on its surface range from 450°F to over 1,000°F. To
withstand this high temperature, the airplane was constructed
of titanium alloy, instead of the traditional aluminum alloy. The supersonic transport Concorde was
originally designed to cruise at Mach 2.2, but its cruise
speed was reduced to Mach 2.0 because of structural
problems that started to occur because of aerodynamic
heating. If airplanes capable of hypersonic flight are
going to be built in the future, one of the obstacles that
will have to be overcome is the stress on the airplane’s
structure caused by heat.